Laser diodes with distributed feedback (also referred to as distributed feedback or DFB laser diodes) are monolithic constructed frequency stabilized laser light sources. In contrast to diode lasers with an external cavity or DBR (Distributed Bragg Reflector) laser diodes, in laser diodes with distributed feedback, the frequency-selective element is monolithically connected to the laser resonator and already interacts with it during radiation generation. The frequency-selective element is usually a Bragg grating in the immediate vicinity of the active layer, which is designed as a surface grating and which interacts directly with the radiation field generated.
The main advantage of laser diodes with distributed feedback is their monolithic construction, by which a compact and stable diode laser structure can be realized. Improvements in design and in material quality have led to immense progress in the implementation of such diode laser concepts in recent years, with the result that optical output powers well above one watt can now be realized in the field of high-power laser diodes by the use of broadarea laser diodes.
In order to achieve the narrowest possible laser emission, a strong interaction of the grating with the optical field is necessary. However, such a strongly interacting grating reduces the efficiency of the outcoupling and increases the optical losses within the resonator. Therefore, a high output power and a high electro-optical conversion efficiency generally oppose a narrow linewidth of the laser diode. The strength of the grating interaction must therefore be adapted very precisely to the laser field in order to achieve an optimum between the two parameters.
In the design of laser diodes with distributed feedback, the influence of the grating must be kept as low as possible, since even a small increase in the electrical resistance of the current path or the occurring optical absorption can often lead to a collapse of the laser performance. Furthermore, the use of gratings of a high order (N≥10) is advantageous since this reduces the requirements for processing, in particular for lithography.
However, in the production of laser diodes with distributed feedback based on such high-order gratings, the allowable manufacturing tolerances for the necessary grating depth generation are extremely small. In addition, it is advantageous if, in the area of high optical power density, the influence of the gratings is reduced in order to reduce the losses in the resonator and to counteract any occurring saturation effects, for example the longitudinal hole burning.
For narrow-band laser diodes comes as another criterion the locking range, which is achievable in operation. The locking range indicates how far apart the grating's Bragg wavelength and the gain maximum of the active layer can be apart without increasing the spectral linewidth (95% power confinement). This distance is also called detuning (Δλdet). A tuning of the locking range is usually done via the temperature of the laser diode. In prior art (Crump et al., J. Phys. D: Appl. Phys. 46 (2013) 013001; Decker et al., IEEE Photon Technol. Lett. 27 (2015) 1675) laser diodes with distributed feedback with uniform gratings for the ratio between optical output line and operating current at room temperature, electro-optical power increases S of 0.9 W/A are typically measured. There, for example, line widths under 1 nm (±2σ range) have been successfully demonstrated in a thermal tuning window of up to 35 K (Δλdet≈10 nm). In the case of uniform gratings, however, the parameter ranges are generally already completely exhausted and largely optimized, so that a further improvement of the thermal tuning behavior can only be achieved at the expense of the conversion efficiency or outcoupling efficiency of the laser diode.
The invention is therefore based on the object to provide a laser diode with distributed feedback, in which one or more of the described problems of the prior art can be avoided or at least significantly reduced. In particular, a laser diode and a method for the production are to be provided, in which the distributed feedback occurs via a surface grating of high order while radiation is outcoupled preferably on one side and in which the coupling strength of the grating is adapted to the power density of the guided wave in the laser diode.